When a light beam is reflected from a sample, its polarization is modified in a way which depends on the structure of the sample, and in particular on its reflection coefficient (rp) parallel to the plane of incidence, and on its reflection coefficient (rs) perpendicular thereto.
Two independent data items are thus obtained, namely the phase difference delta and the amplitude ratio tan psi of the parallel and perpendicular polarizations of the beam reflected from the sample. These ellipsometric parameters are defined by the equation: EQU tan psi e.sup.i.multidot.delta =(rp)/(rs)
where e is the exponential function and i is the unit imaginary vector of complex numbers.
The optical properties of a material are defined by its real index (refractive index n) and its imaginary index (absorption coefficient k). For a thin layer of material, there is a third parameter which characterizes its optical properties: namely its thickness T. In order to characterize a multi-layer sample, three parameters are required per layer, plus the two indices of the substrate. It is then necessary to operate at a plurality of different wavelengths if information is to be obtained on all of the parameters of a multi-layer surface. Further, as the person skilled in the art is well aware, since these measurements are based on a periodic phenomenon, they suffer from ambiguity: the optical period in the layer.
Ellipsometric measurements must therefore be repeated at several different wavelengths. Initially this was done using a laser beam together with a filter for each wavelength.
More recently, spectroscopic ellipsometry has appeared in which the light emitted is taken from a wideband source and a frequency-controlled monochromator is used for separating the various different wavelengths over the entire available spectrum. Such an apparatus is described in an article entitled "Ellipsometrie Spectroscopique" by M..H. Debroux, Ged. A. Vareille, l'Echo des Recherches No. 113, 3rd quarter 1983, pages 61 to 68, and also in a notice from the (French) Centre National d'Etude des Telecommunications (CNET) entitled "Spectroscopic Ellipsometry", edition 20.2.A/CMS, dated 1st March 1984, and published by CNET's Centre Norbert Segard at Grenoble. Reference can be made to these documents for a fuller understanding of spectroscopic ellipsometry.
It also appears that spectroscopic ellipsometry is particularly advantageous for monitoring the manufacture of integrated circuits, in particular during the gas diffusion or ion implanting stages. Spectroscopic ellipsometry can also be used to accurately measure the surface state of a solid, or to investigate a phenomenon relating to such a surface, such as chemical adsorption or absorption.
Thus, a prior art spectroscopic ellipsometry apparatus comprises:
a light source;
a first optical system including a polarizer for illuminating a sample at a sloping incidence with a beam of polarized light which is collimated by a diaphragm;
a second optical system including an analyzer for picking up the light reflected by the sample; and
a photodetector mounted at the outlet from said second optical system.
One of the two polarizing members constituted by the polarizer and the analyzer is subjected to continuous rotation, and in practice to rotation at a constant speed. As a result, the light received by the photodetector is amplitude modulated at a frequency equal to twice the speed of rotation of said member.
Operation is rendered monochromatic either at the light source or else at the photodetector. In spectroscopic ellipsometry, this means that a variable monochromator is located either downstream from the light source or upstream from the photodetector.
Finally, the prior apparatus also includes electronic means for control and processing purposes in order to measure the amplitude of the radiation received by the photodetector as a function of the angle of polarization so as to deduce information concerning the surface state of the sample therefrom in the manner defined above.
In theory, such apparatuses are quite simple. However, they are very difficult to produce in practice, particularly now that laser sources are no longer used and wide band light sources are used instead. Similar remarks can be made about the changeover from discrete optical filters to a monochromator capable of operating continuously over the entire spectrum used.
The main problems encountered are as follows:
Firstly, if the necessary rotation takes place at the polarizer between the light source and the object, any polarization defect in the light source will disturb the measurements. This also applies under opposite circumstances when it is the analyzer which is rotated. In this case, any sensitivity to polarization by the photodetector and in particular any dichroism in its inlet window will degrade measurements.
Secondly, the rotating member, be it the polarizer or the analyzer, produces not only a variable angle of polarization, but also a slight deflection of the beam, thereby causing the beam to follow a circular trajectory. This phenomenon can be compensated at one wavelength only and is therefore incapable of being corrected when a monochromator is used which can be adjusted over a wide optical bandwidth.
The present invention seeks to provide an improved spectroscopic ellipsometer capable of solving the above problems and also of providing other advantages.